1. Field of the Invention
The invention relates to digital hearing aids, and more particularly, to real ear measurement systems for use during hearing aid fitting procedures.
2. Description of Related Art
In present hearing aid fitting systems, the testing methodologies can be considered to be of two different types: either electro-acoustic types of measurements or psycho-acoustic types of measurements. Historically, standardized electro-acoustic measurements have been coupler or real ear based. Coupler-based tests effectively measure some of the electro-acoustic characteristics of the transducers and sound processing of the hearing aid device. The electro-acoustic measurements can also be used to prescribe certain fitting parameters based on gain rules. Real ear measurements have these test capabilities as well. Real ear tests have the added advantage of being in situ rather than test box measurements. Real ear measurements can show the effects of ear canal resonance, head shadow, and venting. With real ear measurements, fitting parameters can be defined using ear canal resonance and targeted in situ gain data.
Even though real ear and coupler measurements provide decibel (Db) sound pressure level (SPL) responses for the fitted hearing device, these measurements do not indicate if the frequency specific amplification delivered in the ear canal is audible or comfortable. For psycho-acoustic test measurements, loudness growth and octave bands (LGOB) measurements are the primary standardized psycho-acoustic test measurements. The need for a clinical measurement of loudness is indicated by "abnormal growth of loudness" characteristics of many patients with sensori-neural and mixed hearing impairments. Such patients exhibit a level and frequency dependent sensitivity to sounds. Because of the patient's reduced dynamic range, the intensity variation of speech and noise are perceived to be exaggerated (in other words, the intensity rises too rapidly from inaudible to soft and from soft to loud). Because loudness sensitivity measurements are highly indicative of the patient's unaided and aided ability to comfortably process loud and soft sounds, LGOB testing allows the clinician to both measure the recruitment of the patient and to use the test results to define fitting parameters.
In these tests, a loud speaker is typically used to deliver the sound source and a probe tube is placed in the ear canal for measuring the sound pressure levels from the sound source. The sound source will typically present an uneven sound field in the room because standing waves that are reflected from the walls will create nulls at approximately 10 centimeters with differences of 20 dB levels. Also, if the user turns his or her head away from the loudspeakers, shadows will be cast which create gain differences of as much as 15 dB. In addition, cost constraints often restrict the size of the testing office, and accordingly a sufficiently large enough distance from the loud speaker to the user may not be accommodated. As a result, these measurements have various degrees of stability and reliability due to the sound presentation.
One known solution for this problem is to make use of a calibrated microphone. In addition to the probe microphone placed in the ear canal, another microphone is placed next to the ear canal entrance at a fixed position. The calibrated microphone is first used to detect the sound presented by the loud speaker and then the microphone registers this level. Thereafter, the calibrated microphone is used to feed back signals for adjusting the sound level of the loud speaker and effectively changing its volume control based upon this registered level. However, the use of such a calibrated microphone undesirably adds additional components and complexity to the system.
During the fitting process of a patient's hearing instruments, the above measurements are made using the microphones of the hearing instrument. Calibration of the hearing instrument is effected using the sound level as presented to the hearing instrument, and then as processed by the hearing instrument and presented to the patient's eardrum. The result is a calibrated instrument which compensates for the variable acoustics of the particular hearing environment.
A drawback of the fitting process is its requirement of complex computing equipment which adds considerable expense and bulk to the fitting process. Additionally, the assembly of the various components and their fitting to the patient during the fitting procedure consumes valuable testing time, inconveniently extending the duration of the fitting process.
Thus there exists a need to simplify the fitting process and make it more convenient and inexpensive for the patient and the dispenser. Such simplification, through a reduction in the amount and size of computation equipment and time required for fittings, increases patients' access to required hearing instruments.